Nowadays, photosensitive compositions usable particularly for high-performance lithographic plates must fulfill high requirements.
In order to improve the properties of photosensitive compositions and thus also of the corresponding lithographic plates, essentially two different ways are taken. One of them deals with the improvement of the properties of the photosensitive components in the compositions (frequently negative diazo resins, photo polymers etc.), the other one with the search for novel polymeric compounds ("binders"), which are to control the physical properties of the photosensitive layers. In particular the latter way is decisive for lithographic plates because the behavior in the developing and printing processes (such as developability, ink receptivity, scratch resistance, consistency in the number of prints produced) is decisively influenced by the polymeric binders. Also shelf life and photosensitivity of the materials are strongly influenced by such polymeric compounds.
The polymeric binders, therefore, exhibit various structural elements for satisfying the extensive requirements, which may have different effects on individual properties. For instance, hydrophilic structural elements such as carboxyl groups, hydroxyl groups and the like generally promote the developability of the photosensitive compositions in aqueous alkaline developers and partly ensure sufficient adhesion to polar substrates. Hydrophobic structural elements, on the other hand, reduce the capability of being developed in the above-mentioned developers, but ensure the good ink receptivity used in the printing process, which is indispensable in lithographic plates.
Due to the broad range of requirements regarding the polymeric binders, there have been extensive studies for many years on the synthesis and optimization of the use of these substances for photosensitive compositions, cf. e.g. H. Baumann and H. -J. Timpe: "Chemical Aspects of Offset Printing" in J. prakt. Chem./Chemiker-Zeitung [Journal for chemists] 336 (1994) pages 377-389.
EP-A-135 026, EP-A-186 156 and U.S. Pat. No. 4,731,316 describe binder systems consisting of compositions of polymers having different hydrophilic/hydrophobic properties. However, such compositions have the disadvantage that very frequently incompatibilities between the different substances lead to separation during the formation of layers. Furthermore, it was found that the hydrophobic polymers precipitate during the developing process of the processing of the lithographic plates, which may lead to silting in the developing machines and the redeposition of dispersed layer portions on the surface of the photosensitive material.
Furthermore, various copolymers consisting of only slightly hydrophilic monomers such as styrene, acrylic acid ester, methacrylic acid ester and the like with hydrophilic comonomers were described. Examples of such comonomers are semi-esters of maleic acid (DE-A-31 30 987, EP-B-71 881, EP-A-104 863), itaconic acid (EP-A1-397375, U.S. Pat. No. 5,260,161) and acrylic acid and/or methacrylic acid (EP-A-487 343, U.S. Pat. No. 4,304,832, U.S. Pat. No. 4,123,276). The very tight play of the properties important for their use, which are layer adhesion, developability and printing ink receptivity, proved to be disadvantageous in such polymers. Variations in the polymers' composition can hardly be avoided during the production process, which leads to unacceptable fluctuations in the plates' properties.
DE-A-27 51 060 describes photosensitive compositions, wherein the binder is a reaction product of cellulose esters with cyclic, intermolecularacid anhydrides of dicarboxylic acids. These binders, however, are not oleophilic enough for the use in lithographic plate formulations.
Polymers that contain urethane groups were also described as binders for photosensitive compositions (EP-A2-415 302, EP-A1-406 599, EP-A1-406 600, EP-A2-414 099, U.S. Pat. No. 4,983,491, U.S. Pat. No. 4,950,582, U.S. Pat. No. 4,877,711). These polyurethane resins contain functional groups (--COOH, --SO.sub.2 NHCOO--, --CONHSO.sub.2 NH--) with an acidic hydrogen, whose pK.sub.a values in water are no higher than 7. The necessary functionalization with such hydrophilic groups, however, requires very high efforts regarding synthesis and involves high costs.
Another group of binders described are acetals of aliphatic aldehydes having a lower alkyl group and vinyl alcohol/vinyl acetate copolymers (U.S. Pat. No. 2,179,051, EP-B1-216 083). Such binders, however, cause problems in the developing of lithographic plates due to the portion of hydrophilic groups in the polymer being too low. Semi-acetalized vinyl alcohol/vinyl acetate copolymers were provided with alkali soluble groups by means of special reactions for improvement (EP-A-48 876, U.S. Pat. No. 4,387,151, U.S. Pat. No. 3,732,106, DE-A-20 53 363, EP-A-152 819, DE-A-36 44 163, U.S. Pat. No. 4,741,985, EP-A-208 145, DE-A-37 01 626, U.S. Pat. No. 5,169,897, DE-A-36 44 162, U.S. Pat. No. 4,940,646, DE-A-39 03 001 and U.S. Pat. No. 5,219,699). Particularly preferred, carboxyl and sulfonyl urethane groups were introduced into the polymers in order to achieve the desired developability parameters. The introduction of the acidic carboxyl group into the side chain of the polyvinyl acetals may, however, lead to ink receptivity problems, particularly when alkaline wetting agents arc used in the printing process. In the case of sulfonyl urethane containing binders, this disadvantage is almost excluded; however, the production of such binders involves difficulties and high costs. Also in the case of polyvinyl acetals, which have stilbazole units within the molecule (U.S. Pat. No. 5,330,877), a good ink receptivity is practically not achieved.
In addition, all these known systems have the disadvantage of a relatively low photochemical sensitivity, which is why they are rather unsuitable for applications requiring highly sensitive compositions.
It is known that the light-induced cross-linking of imido-group containing polymers in the presence of certain sensitizers may lead to photosensitive compositions having a higher photochemical sensitivity. However, their use for producing lithographic plates is strongly limited due to an insufficient adhesion on the commonly used aluminum substrates. This particularly leads to a reduced number of prints produced, so that their practical use is complicated greatly. Therefore, especially prepared substrates are necessary (EP-A-377 589), or diazo resins (U.S. Pat. No. 5,112,743, DE-A-3 633 456, EP-A-368 327) and/or certain polymers (EP-A-679 950) must be added. The latter measures, however, lead to a lower sensitivity of such compositions, which in turn leads to longer exposure times.